What kind of monitor are you using? Is this a Laptop?
I found this on an FAQ website:
1. Bad Firmware: The first cause is outdated firmware, combined with the use of OS 10.2.x (Jaguar). This can cause all manner of wacky screen behavior, from dim, discoloured screens, to outright blank, black screens. The problem may come and go, or get progressively worse. The solution is simple. Download the latest firmware, from here: http://docs.info.apple.com/article.html?artnum=75130 You can check what firmware your iMac has in Apple System Profiler (located in /Applications/Utilities). The ASR program's Boot ROM Info field tells you your firmware revision. As of this writing, the latest version is 4.1.9. You'll have to install the firmware update while booted from a write-enabled OS 9 disk (no CD-booting). If you can't see what's happening on your monitor, you might have to get creative, using Timbuktu or the VGA out port to see what's happening.
2. Badly Adjusted Cathode Ray Tube: The second major cause of unsatisfactory screen performance is a maladjusted CRT. Symptoms are usually less severe than with bad firmware, but are highly annoying nonetheless. They include fuzzy, out of focus areas on the screen (sometimes the at center but not the sides, or vice versa), excessively dark screens, or excessively bright, washed out screens. You may notice, while watching a DVD, that parts of a scene that should be in shadow, but still clear, are instead completely black. Or you may notice that even with the monitor brightness set to the max, digital photos appear dark and underexposed on the iMac screen, but not on other computers. Brightness (voltage) and focus are adjusted via two screws on the high voltage flyback transformer on the left side of the iMac, just below the edge of the upper plastic shell. The adjustment is not difficult, but should be performed with extreme caution. Even when the monitor is powered off and unplugged, it still holds enough charge to kill you instantly. IF YOU ARE UNCLEAR OR UNCERTAIN OF WHAT YOU ARE DOING, STOP! TAKE IT TO A CERTIFIED APPLE REPAIR TECHNICIAN.
If you are confident that you can handle it, do the following:First, you will want to download the appropriate repair manual PDF. Also, download the Display Service Utility 4.2.1 from the Utilities section, further down that page.The manual mentions the use of a special monitor adjustment jumper cable. However, despite what the manual says, this is not necessary for basic brightness and focus adjustments. The more complicated adjustments of the color guns etc, which do require the jumper, are best left to a professional. You are advised not to touch ANY parts of the CRT's internals.
To access the voltage and focus adjustment screws, you will need to remove the lower plastic casing of the iMac. This is explained, with illustrations, in the PDF manual. You just need to remove the plastic, not the metal mesh EMF sheilding. Also, use the manual to familiarize yourself with the location of the voltage and focus adjustment screws on the flyback transformer.
Once you've removed the lower casing, place the iMac rightside up again, on a wooden or otherwise nonconductive surface. It should sit stably on its internal metal frame. Plug in the power, keyboard, and mouse, and boot into OS 9. Be careful not to touch any exposed monitor or analog circuitry.
In OS 9, make sure the screen is running at 800x600 resolution, with brightness set to medium, and contrast set to the max. Then run the Display Service Utility. Click on the first standard pattern. Your screen should show a series of vertical grey bars. You will use these to calibrate brightness. Using a Phillips or slot head screwdriver with a PLASTIC handle, carefully turn the voltage adjustment screw (the lower one) until you can just barely see the difference in brightness between the two leftmost (darkest) vertical bars. When the brightness is properly adjusted, you should be able to see the difference in brightness between all of the bars on the screen. No two bars should look the same.
Once brightness is adjusted, you can adjust focus, if necessary. Click on the rightmost standard pattern in the Display Service Utility. You should get a focus test pattern on the screen. Carefully adjust the focus screw (the upper one) so that both the center and edge test patters are reasonably crisp. You might have to find a compromise between center and edge clarity, but it should be possible to get it pretty good.
Be careful to only touch the plastic handle of your screwdriver, not the metal shaft, and not to let the screwdriver touch any metal parts except for the screws themselves.
That's it. Shut down the iMac, unplug the cables, and carefully replace the lower plastic shell. You're done!
Now, if this is not an option for you, you can locate an apple repair in your city, mostly likely by searching whitepages.com. If you want to tell me what city and state you are in I can try to find one for you. I think it would probably be costly to repair, but I am not sure. I am online, so let me know, I am here to help!